FIG. 1 is a side view of part of a fastener assembly passing through a panel 1, which may be a composite or metallic panel. The assembly comprises a fastener comprising an externally threaded bolt 2, an internally threaded nut 3, and a washer 4 (the fastener may alternatively comprise any other known fastener type, such as a rivet or swage fastener). In the event of a lightning strike hitting the panel 1 and attaching to the fastener, sparking, plasma or out-gassing may occur at the locations indicated by reference 5 in FIG. 1. The panel 1 may provide a fuel tank boundary and the fastener may therefore be immersed in fuel. A lightning strike at the fastener may thus provide sparking and hot gas ignition sources which could cause ignition of the fuel.
A known method of providing spark suppression is described in EP-A-0334011. A volume of gas is enclosed by a cap around the fastener. The gas provides spark suppression for arcing that may occur between the composite structure and the metal fastener during any lightning strike.